1. Field of the Invention
The present invention relates to an optical amplifier, an optical amplification apparatus using such an optical amplifier, and an optical amplification method used in such an optical amplifier and such an optical amplification apparatus. The present invention is suitable to be applied to various field in which light having high output is required, particularly, fields such as optical fiber communication, satellite-to-satellite communication and the like in which high output optical signal is required, a laser marker field, medical and biological fields to which non-linear optics are applied, and a semiconductor manufacturing field.
2. Related Background Art
In the past, an electronic repeater having 3R (reshaping, regenerating and retiming) functions has been used for amplifying an optical signal. However, when an optical signal subjected to wavelength-division-multiplexion for achieving large capacity is amplified by the electronic repeater having 3R function, since it is required that combined light be once divided and the divided lights be amplified and then are combined again, the system becomes very complicated and expensive. Further, when optical signal is transferred at a very high speed, in the electronic repeater having 3R functions, if the speed of the optical signal is changed (for example, changed from 10 MHz to 10 GHz), the electronic repeater having 3R functions itself must be changed.
Thus, as large capacity and high speed long distance communication has been developed, an optical fiber amplifier has been proposed as an amplifier for solving the problems regarding the electronic repeater having 3R functions. As shown in FIG. 9A, in the optical fiber amplifier, when optical signal having a single mode is inputted to a single mode fiber A and a single mode pumping light (laser beam) generated from a pumping source (semiconductor laser) B is combined with the optical signal by a wave combining unit C, the combined light is transferred to an optical amplifier fiber D, where amplifying medium included in the optical amplifier fiber D is pumped by the pumping light, with the result that the optical signal is subjected to optical amplification by stimulated emission.
In order to give higher output to the optical signal by the said optical fiber amplifier, a high output pumping source must be used to input high output pumping light to the amplifying medium. However, the pumping light outputted from the single pumping source has limitation, as shown in FIG. 9B, a plurality of pumping sources B are prepared, and pumping lights from the respective pumping sources B are combined together by wave combining units E to obtain high output, and the combined pumping light is combined with the optical signal by a wave combining unit C.
An optical signal amplifying method utilizing the optical fiber amplifier shown in FIG. 9A has the following disadvantage. That is to say, size of a waveguide is limited to propagate the optical signal in a single mode. Namely, it allows a propose multi mode where transit signal is scramble to multi mode. On the other hand, if the waveguide is narrow, light power density is increased as the optical signal is amplified, thereby increasing distortion of the optical signal due to non-linear effect. After all, in the optical fiber amplifier in which the amplifying medium is included in the waveguide, the optical signal cannot be amplified more than a certain limit output.
An optical signal amplifying method utilizing the optical fiber amplifier shown in FIG. 9B has the following disadvantage. That is to say, when the pumping lights from the plurality of pumping sources are combined steppingly to obtain the pumping light having high output, energy loss generated each wave combination is increasingly accumulated, with the result that many parts are required for achieving the high output. Thus, the cost is increased and high energy is required, and, in actual, it is difficult to obtain the pumping light having high output greater than 2 Watts.
Therefore, an object of the present invention is to provide a high output optical amplifier in which distortion property and energy loss can be reduced as less as possible, and an optical amplification apparatus using such an optical amplifier, and an optical amplification method used in such an optical amplifier and such an optical amplification apparatus.
The present invention aims to achieve the above object by utilizing a geometrical feature in which light emitted from a light source positioned on one of foci of an ellipse is always focused on the other focus thereof. A light collecting technique such a geometrical feature has already been used on a solid-state laser. More specifically, as shown in FIG. 10, an amplifying medium (ruby crystal) G is positioned on one of focal axes of an elliptical cylindrical mirror F and a flash lamp H (to which an electric power is supplied from a power supply J) is positioned on the other focal axis thereof so that light outputted from the flash lamp H is effectively incident on the amplifying medium G in such a manner that the incident light is resonated in the amplifying medium G to increase the output above a predetermined threshold value.
Now, the present invention will be described more concretely.
According to a first aspect of the present invention, there is provided an optical amplifier comprising a substrate having a surface disposed on or adjacent to a focal axis passing through one of foci of an elliptical cylinder or an elliptical cone mainly made of SiO2-glass or crystal and extending in an optical signal propagating direction, an amplifying medium disposed on or adjacent to the other focal axis of the substrate, and a pumping source unit disposed on or adjacent to the surface of the substrate.
According to a second aspect of the present invention, in the optical amplifier according to the first aspect, reflection coating for reflecting pumping light is applied to surfaces other than a pumping light input inlet and both end faces of the amplifying medium which are optical signal input and output end faces, among outer surfaces of the substrate, and films capable of reflecting the pumping light and permitting passage of the optical signal are coated on the both end faces of the amplifying medium.
According to a third aspect of the present invention, in the optical amplifier according to the first or second aspect, the pumping source unit includes a plurality of pumping sources, and pumping density of the amplifying medium can be adjusted and controlled by changing distances between the pumping sources or by using pumping sources having different outputs or by making outputs of the pumping sources externally-controllable or by using self-controllable pumping sources.
According to a fourth aspect of the present invention, in the optical amplifier according to any one of the first to third aspect, a plurality of pumping sources are used in the pumping source unit so that, if any pumping source cannot emit predetermined output, such a pumping source can be replaced by a new one.
According to a fifth aspect of the present invention, in the optical amplifier according to any one of the first to fourth aspect, when it is assumed that a length of the substrate is L, a longer radius of the substrate is a, a distance between a center O and a focus F of the substrate is OF, a wavelength of optical signal is xcex, a minimum spot size of the optical signal is w0, a maximum width of the amplifying medium 3 is 2r, and a relationship between the minimum spot size w0 of the optical signal and the maximum width 2r of the amplifying medium 3 is w0=r, the substrate and the amplifying medium is selected to satisfy the following relationship:
2xcexxc3x97(L/2)/xcfx80w0xe2x89xa6axe2x88x92OF.
In an optical amplification apparatus according to the present invention, a lens collimating the optical signal to be incident on the amplifying medium is arranged in front of the optical signal input end face of the amplifying medium, whereby the optical signal is propagated through the amplifying medium not as a waveguide but as a propagating medium (light propagating path).
In an optical amplification method according to the present invention, the optical signal as free spatial light is propagated through the amplifying medium.
The amplifying medium in the said optical amplifier may be mainly formed from SiO2-glass or crystal and, it is desirable that rare earth element is mixed therewith as amplifying medium. In this case, a cross-sectional area of the amplifying medium is sufficiently greater than that of the optical fiber not to establish high density even when the optical signal is amplified, thereby preventing the distortion property of the optical signal due to non-linear effect form being increased.
It is desirable that oscillation preventing treatment is applied to the input and output end faces of the amplifying medium of the said optical amplifier to prevent oscillation of the optical signal in the amplifying medium. In this case, since the optical signal is not oscillated within the amplifying medium, amplification is stably realized with high efficiency.
It is desirable that the input and output end faces of the amplifying medium of the said optical amplifier are inclined with respect to the optical signal transferring direction. In this case, the optical signal incident on the amplifying medium is not oscillated within the amplifying medium.
It is desirable that constituents of the substrate and the amplifying medium of the saida optical amplifier are adjusted and controlled so that there is no difference in index of refraction between them. In this case, loss of pumping efficiency due to influence of reflection and/or refraction does not occur at the interface between the substrate and the amplifying medium.
As the pumping source of the said optical amplifier, high output LD, multi-mode broad stripe LD, Master Oscillator Power Amplifier (MOPA) or LD array can be used. In this case, pumping ununiformity can be suppressed more minutely.
In order that the said optical amplifier can provide stable output without malfunction due to heat, it is desirable that the substrate is provided with a radiator mechanism.
It is desirable that the lens of the said optical amplification apparatus can change the optical signal to parallel light having extension substantially the same as a cross-sectional area of the amplifying medium to be incident on the latter. In this case, since the cross-sectional area of the amplifying medium can be utilized at the maximum, the non-linear effect due to increased in light power density is more hard to be occur.